support for viewParams in WFS GetFeature POST requests

Support for the viewParams vendor parameter in WFS GetFeature requests for WFS 1.0/1.1/2.0.  Fixes GEOS-5160: http://jira.codehaus.org/browse/GEOS-5160

Changes:
* Removed viewParmams from metadata map for KVP requests
* Store the viewParams in the GetFeature model instead
* Update the WFS 1.0/1.1 GetFeature Binding classes to read the model when building requests
* Write unit tests to perform viewParams POST requests

Depends on:
* https://github.com/geotools/geotools/pull/165 to update the model for WFS GetFeature